Field of the Invention
The invention relates to an article of manufacture having a metal substrate, an oxide layer and an anchoring layer disposed therebetween. More specifically, the invention relates to a superalloy substrate, a thermal barrier layer and a layer anchoring the thermal barrier layer onto the substrate. The invention also relates to a method for bonding the article of manufacture, the oxide layer and the anchoring layer.
U.S. Pat. No. 4,055,705 to Stecura et al.; U.S. Pat. No. 4,321,310 to Ulion et al.; and U.S. Pat. No. 4,321,311 to Strangman disclose coating systems for gas turbine components made from nickel or cobalt-based superalloys. A coating system described therein includes a thermal barrier layer made from ceramic, which in particular has a columnar grained structure, that is placed on a bonding layer or bond coating which in turn is placed on the substrate and bonds the thermal barrier layer to the substrate. The bonding layer is made from an alloy of the MCrAlY type, namely an alloy containing chromium, aluminum and a rare earth metal such as yttrium in a base including at least one of iron, cobalt and nickel. Further elements can also be present in an MCrAlY alloy, with examples being given below. An important feature of the bonding layer is that a thin oxide layer of alumina or a mixture of alumina and chromium oxide, depending on particulars of the composition of the MCrAlY alloy, inter alia, is developed on the MCrAlY alloy in an oxidizing environment below the thermal barrier layer.
Accordingly, a bond between the thermal barrier layer and the alumina layer must be provided.
U.S. Pat. No. 5,238,752 to Duderstadt et al. discloses a coating system for a gas turbine component which also incorporates a ceramic thermal barrier layer and a bonding layer bonding the thermal barrier layer to the substrate. The bonding layer is made from an intermetallic aluminide compound, in particular a nickel aluminide or a platinum aluminide. The bonding layer also has a thin oxide layer which must serve to anchor the thermal barrier layer.
U.S. Pat. No. 5,262,245 to Ulion et al. describes a result of an effort to simplify coating systems incorporating thermal barrier layers for gas turbine components by avoiding bonding layers. To that end, a composition for a superalloy is disclosed which may be used to form a substrate of a gas turbine component and which develops an alumina layer on its outer surfaces under a suitable treatment. That alumina layer is used to anchor a ceramic thermal barrier layer directly on the substrate, eliminating the need for a special bonding layer to be placed between the substrate and the thermal barrier layer.
U.S. Pat. No. 5,087,477 to Giggins et al. shows a method for placing a ceramic thermal barrier layer on a gas turbine component by a physical vapor deposition process. That process includes establishing an atmosphere having a controlled content of oxygen at the component to receive the thermal barrier layer, evaporating compounds with an electron beam and forming a gas phase, and precipitating the gas phase on the compound to form the thermal barrier layer.
U.S. Pat. Nos. 5,154,885; 5,268,238; 5,273,712; and 5,401,307 to Czech et al. disclose advanced coating systems for gas turbine components including protective coatings of MCrAlY alloys. The MCrAlY alloys disclosed therein have carefully balanced compositions in order to provide exceptionally good resistance to corrosion and to oxidation, as well as an exceptionally good compatibility (mechanical, chemical) to the superalloys used for the substrates. The basis of the MCrAlY alloys is formed by nickel and/or cobalt. Additions of further elements, in particular silicon and rhenium, are also discussed. Rhenium in particular is shown to be a very advantageous additive. All MCrAlY alloys shown are also very suitable as bonding layers for anchoring thermal barrier layers on gas turbine components. U.S. Pat. No. 5,401,307 also contains a survey of compositions of superalloys which are useful for forming gas turbine components.
A standard practice in bonding an oxide layer, in particular a thermal barrier layer, to an article of manufacture, is placing an anchoring layer formed of alumina on the article, either by placing a suitable bonding layer on the article which develops the alumina on its surface under oxidizing conditions or by selecting a material for the article which is itself capable of developing alumina on its surface. The thermal barrier layer is placed on the bonding layer and bonded to the substrate through the anchoring layer.
The thermal barrier layer itself is expediently made from an oxide ceramic, particularly stabilized or partly stabilized zirconia. "Partly Stabilized Zirconia" classifies a family of preparations containing zirconia as a principal constituent and at least one other compound which is thoroughly mixed with the zirconia and which inhibits the zirconia to change its crystalline properties under thermal cycling. Examples of such other compounds are yttria, calcia, magnesia, ceria and lanthana. In order to produce the desired effect, those other compounds have to be admixed to the zirconia in amounts up to 10 percent by weight or even more. Examples are available from the cited documents of the state of the art.
Particulars of the bonding of the thermal barrier layer to an anchoring layer formed essentially of alumina have not yet attained considerable recognition or even discussion from the technicians working in the field. It has more or less been taken for granted that the alumina, being a ceramic itself and being developed as a layer bonded to a suitable metal substrate, would assure bonding between the metal substrate and a thermal barrier layer placed upon the alumina by its mere existence. Heretofore, only an alumina layer has been given consideration to anchor a thermal barrier layer on a superalloy substrate regardless of its declining bonding capability and increasing spalling probability as it grows during operation due to the exposure to an oxidizing environment. Such growth must be expected to occur at a gas turbine component under high thermal load in an oxidizing environment.
Further inquiries by the inventor have revealed that it is not the mere existence of an alumina anchoring layer which bonds a thermal barrier layer placed thereon thereto, but that it is a solid state chemical reaction occurring between the alumina and the thermal barrier layer which creates a thin mixing zone between the anchoring layer and the thermal barrier layer where compounds formed from both the anchoring layer and the thermal barrier layer provide for bonding.